This proposal is concerned with the conformational properties of DNA under very low salt conditions. The left-handed, Z form of poly(dG-dC) and ply(dG-m5dC) exists in solutions containing high salt or oligovalent cations and the right-handed, B form exists under low salt conditions. We have recently observed the unexpected result that in very low salt solutions (less than 2mM Na+) poly(dG-m5dC) is again left-handed. Hence, as the ionic strength is increased above millimolar levels, poly(dG-m5dC) is first left-handed, then right-handed, and finally left-handed again. The research is designed to delineate the solution conditions which stabilize the low salt, left-handed form of poly(dG-m5dC) using circular dichroism, to analyze equilibrium and kinetic aspects of the low salt left-handed form to B form transition, to characterize its conformation, in particular relative to that of the high salt Z form, using 1H and 31P NMR, to examine (dG-m5dC)n oligonucleotides and other sequences for this effect, to investigate the H-D exchange properties of the low salt form using hand mixing and stopped-flow techniques, and to measure the relative rise per base pair in the low salt and intermediate salt forms of poly(dG-m5dC) with transient electric birefringence. The ability to exist in a left-handed conformation at both low and high salt increases the conformational lability of poly(dG-m5dC). Since dG-m5dC sequences have been implicated in gene expression, this conformational flexibility may play an important biological role.